Fiber-optic communication is widely used in applications such as telecommunications and communication within large data centers. Because of attenuation losses associated with using shorter optical wavelengths most fiber-optic communication uses optical wavelengths of 800 nm and longer. Commonly used transmission windows exist between 800 nm and 1675 nm. A main component of optical receivers used in fiber-optic communication system is the photo detector, usually in the form of a photodiode (PD) or avalanche photodiode (APD).
High-quality low-noise APDs can be made from silicon. However, while silicon will absorb light in the visible and near infrared range, it becomes more transparent at longer optical wavelengths. Silicon PDs and APDs can be made for optical wavelengths of 800 nm and longer by increasing the thickness of the absorption “I” region of the device. However, in order to obtain adequate quantum efficiency, the thickness of the silicon “I” region becomes so large that the device's maximum bandwidth is too low for many current and future telecom and data center applications.
To avoid the inherent problem that silicon PDs and APDs have with longer wavelengths and higher bandwidths, other materials are used. Germanium (Ge) detects infrared out to a wavelength of 1700 nm, but has relatively high multiplication noise. InGaAs can detect out to longer than 1600 nm, and has less multiplication noise than Ge, but still has far greater mulitiplacation noise than silicon. InGaAs is known to be used as the absorption region of a heterostructure diode, most typically involving InP as a substrate and as a multiplication layer. This material system is compatible with an absorption window of roughly 900 to 1700 nm. However, both InGaAs devices are relatively expensive and have relatively high multiplication noise when compared with silicon and are difficult to integrate with Si electronics as a single chip.
Information published by a major company in the business of photodetectors (See http://files.shareholder.com/downloads/FNSR/0x0x382377/0b3893ea-fb06-417d-ac71-84f2f9084b0d/Finisar_Investor_Presentation.pdf) indicates at page 10 that the current market for optical communication devices is over 7 billion U.S. dollars with a compounded annual growth rate of 12%. The photodiodes (PD) used for 850 nm wavelength employ GaAs material and for 1550 nm wavelength the photodiodes are InP material based, which is both expensive and difficult to integrate with Si based electronics. Therefore, there is a large market and a long-felt need that has not met for the development of a better device. To date there are no Si material based photodiodes nor avalanche photodiodes (APD) for 850 nm and no Ge on Si material based photodiodes nor avalanche photodiodes for 1550 nm that are top or bottom illuminated and with data rate of 5 Gb/s or greater, that are commercially available to the knowledge of the inventors herein. However, there has been no lack of trying to develop a better device for this large market. For example, there have been proposals for resonant photodiodes fabricated in Si material (see Resonant-Cavity-Enhanced High-Speed Si Photodiode Grown by Epitaxial Lateral Overgrowth, Schaub et al, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 12, DECEMBER 1999), but they have not reached the known commercial market. Other forms of high speed photodiodes in a waveguide configuration have been proposed, such as in Ref. 40 GHz Si/Ge uni-traveling carrier waveguide photodiode, Piels et al, DOI 10.1109/JLT.2014.2310780, Journal of Lightwave Technology; Monolithic Ge/Si Avalanche Photodiodes, Kang et al, 978-1-4244-4403-8/09/$25.00 ©2009 IEEE; High-speed Ge photodetector monolithically integrated with large cross-section silicon-on-insulator waveguide, Feng et al, Applied Physics Letters 95, 261105 (2009); doi: 10.1063/1.3279129; where light is coupled edge-wise into an optical waveguide and where the absorption length can be 100 um or longer to compensate for the weak absorption coefficient of Ge at 1550 nm. In these previously proposed waveguide photodiode structures, light propagates along the length of the waveguide and the electric field is applied across the PIN waveguide such that the direction of light propagation and the direction of the electric field are predominately perpendicular. Since light in Si travels approximately 1000 times faster than the saturated velocity of electrons/holes, a waveguide PD can be 200 microns long for example and the “I” in the PIN can be 2 microns for example and achieve a bandwidth of over 10 Gb/s. Such edge coupling of light is costly in packaging as compared to surface illumination as described in this patent specification, where dimensions of the waveguide cross section are typically a few microns as compared to tens of microns for known surface illuminated photodiodes or avalanche photodiodes. Known waveguide PD/APD are often only single mode optical systems whereas surface illuminated PD/APD described in this patent specification can be used in both single and multimode optical systems. In addition, known waveguide photodiodes are difficult to test at wafer level, whereas surface illuminated photodiodes described in this patent specification can be easily tested at wafer level. Known waveguide photodiodes/avalanche photodiodes are used mostly in specialty photonic circuits and are not widely commercially available. A top or bottom illuminated Si and Ge on Si PD/APD that can be integrated with Si is not known to be commercially available at data rates of 5 Gb/s or more at wavelengths of 850 nm and 1550 nm. In contrast, photodiodes on Si based material, as described in this patent specification, can be monolithically integrated with integrated electronic circuits on a single Si chip, thereby significantly reducing the cost of packaging. In addition, the microstructured PD/APD at 850 nm nominal wavelength described in this patent specification can be predominately for short haul, distances less than a meter and in certain cases less than 10 meters and in certain cases less than 100 meters and in certain cases less than 1000 meters optical data transmission for example. The microstructured PD/APD direction of incident optical beam and the electric field in the “I” region of a PIN or NIP structure, are predominately collinear and or almost collinear. This patent specification enables such a device and is expected to transform the current data centers to almost all optical data transmission between blades and or within a blade, that will vastly increase the data transmission bandwidth capabilities and significantly reduce electrical power usage.
The subject matter claimed herein is not limited to embodiments that solve any specific disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.